Re: [PATCH 15/28] mm, page_alloc: Move might_sleep_if check to the allocator slowpath

[Vlastimil Babka]

On 04/15/2016 11:07 AM, Mel Gorman wrote:
> There is a debugging check for callers that specify __GFP_DIRECT_RECLAIM
> from a context that cannot sleep. Triggering this is almost certainly
> a bug but it's also overhead in the fast path.

For CONFIG_DEBUG_ATOMIC_SLEEP, enabling is asking for the overhead. But for 
CONFIG_PREEMPT_VOLUNTARY which turns it into _cond_resched(), I guess it's not.

> Move the check to the slow
> path. It'll be harder to trigger as it'll only be checked when watermarks
> are depleted but it'll also only be checked in a path that can sleep.

Hmm what about zone_reclaim_mode=1, should the check be also duplicated to that 
part of get_page_from_freelist()?

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>   mm/page_alloc.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 21aaef6ddd7a..9ef2f4ab9ca5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3176,6 +3176,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>   		return NULL;
>   	}
>
> +	might_sleep_if(gfp_mask & __GFP_DIRECT_RECLAIM);
> +
>   	/*
>   	 * We also sanity check to catch abuse of atomic reserves being used by
>   	 * callers that are not in atomic context.
> @@ -3369,8 +3371,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>
>   	lockdep_trace_alloc(gfp_mask);
>
> -	might_sleep_if(gfp_mask & __GFP_DIRECT_RECLAIM);
> -
>   	if (should_fail_alloc_page(gfp_mask, order))
>   		return NULL;